The present invention relates to monitoring damage to subsea equipment. In a particular aspect the present invention relates to monitoring fatigue in subsea riser strings in real time.
The prediction and monitoring of fatigue damage resulting from vortex-induced vibrations (VIV) of drilling risers is a complex and a challenging problem in deepwater drilling environments. Although multiple sources of fatigue damage exist, VIV and waves are the primary causes of fatigue damage to deepwater drilling risers. Undersea currents can result in VIV in which the drilling riser vibrates in a direction perpendicular to the dominant current direction. Unlike shallow environments, deepwater drilling requires relatively high top tension to maintain lateral stability of the riser string. This high tension in combination with stresses produced by strong currents may result in components of the subsurface installation served by the riser string (e.g. BOP-stack-conductor) vibrating at or near a component resonant frequency and lead to in increased rates of fatigue damage and increased susceptibility of the overall system to fatigue failure.
At present, drilling riser monitoring systems use vibration data loggers that provide data on stresses experienced along the riser string after the loggers are recovered at the end of a drilling campaign. Real time data is generally not available with which to continuously assess damage being accumulated along the length of the riser. As a result, assessment of fatigue damage occurring during a drilling campaign often relies upon predictive models applied before the drilling campaign is begun. In the face of such uncertainty, damage rate estimates are relatively conservative and tend to exceed actual damage rates, thereby limiting both riser life and riser operational flexibility.
Thus, there is a need for systems and methods for reliably determining damage rates in marine risers in real time. The present invention provides new systems and methods which address one or more of the aforementioned problems.